Relational algebra toolkit

Extended relational algebra (aggregation
and grouping)

δ is the duplicate elimination operator (= SELECT DISTINCT)

γ is the grouping operator

example (courtesy [FCDB] example 5.23, p. 225): consider
relation schema StarsIn(title, year, starName) and
query for each star who has appeared in a least three movies,
the earliest year in which they appeared: